1. Field of the Invention
The present invention relates generally to electronic aids to navigation, and more specifically to (simulated) Doppler Scanning systems for aircraft landing approach guidance.
2. Description of the Prior Art
In the prior art the so-called Doppler navigation systems are extensively described in the technical and patent literature. The basic system to which the present invention applies is well described in the technical paper "Doppler Scanning Guidance System" by C. W. Earp, F. G. Overbury and P. Sothcott, published in the periodical "Electrical Communication", Vol. 46 (1971), No. 4, pp. 253 to 270.
Basically the aforementioned prior art systems utilize a ground transmitter and a linear antenna having plural separately-excited radiation elements, the signal transmitted being applied successively by a commutator to the radiator elements to simulate, for a reception point located at a distance therefrom, an antenna moving along the axis of the array. That arrangement is commonly called the Doppler (ground) Beacon.
The bearing (or elevation angle) with respect to the array is defined by the frequency of the commutated wave emitted in that direction, i.e. the angular information is coded in terms of frequency.
A remote receiver of the system includes a frequency selective network for tracking, i.e. acquiring and locking on to, the major frequency spectrum of the received signal. After certain signal processing via r.f. and detector stages, a Doppler beat frequency signal is derived and is indicative of the angle.
One of the most serious problems affecting the accuracy of angle determination in systems of the aforementioned type is the problem of multi-path signals reaching the remote receiver. The process of developing the so-called beat, and therefrom its analog, which is the desired angle information, is an air-derived process. Accordingly, the angle-coded ground Doppler Beacon signals may be received, not only directly by line-of-sight, but also by reflection from the ground or various manmade or natural obstacles near the approach path.
Under most conditions, these multipath components are sufficiently well spaced from the main angle information component for the frequency selectivity of the tracking filter to substantially eliminate them and therefore all errors caused by the multipath components. However, there are conditions, especially relevant to elevation systems, where a multipath component is close in frequency to that of the desired direct signal and of an amplitude approaching that of the desired signal, e.g. relative amplitude 0.7 to 1.
In elevation Dopper systems, since the transmission is in the form of a fan beam, a reflecting object, such as a hangar, at the side of the runway, although at a relatively wide angle in azimuth from the glidepath, may cause a multipath component to occur which is of relatively high amplitude and as close as 1/2.degree. angular separation vis-a-vis the true (direct path) elevation angle.
In an aircraft borne system receiver, flying along the glidepath, the relative path length changes causing cycling between peak positive and peak negative errors. Although the movement of the receiver is ordinarily fast enough to smooth out the instantaneous errors, the mean error is not zero, and this results in an error being introduced into the count made in frequency tracking circuits.
The manner in which the present invention deals with this problem will be understood as this description proceeds.